How to match a corresponding css from a point in html?

I wanna achieve something like LiveCSS in Calibre: When placed a cursor in html source, get the corresponding css style of that point. Any suggestions? It's like the thing in the screenshot. The selected text is on the left pane, corresponding CSS rules on the right. I want to do a similar thing in Python - given a point in a string containing HTML source, return CSS style rules for that point.